System and method for cleaning in-process sensors

ABSTRACT

A cleaning system and method for in-process sensors wherein a scouring jet discharges process fluid as the cleaning agent to remove solids and other contaminants from the surface of the sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation claiming the benefit of U.S. patentapplication Ser. No. 10/256,672, filed Sep. 27, 2002, which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to an improved system and method for cleaning ofin-process sensors.

BACKGROUND OF THE INVENTION

In-process sensors, such as on-line sensors, are used widely in thechemical, pharmaceutical, and food processing industries to measure oneor more characteristics, such as composition, temperature, pressure, orpH of a process fluid. All of these sensors have a surface by which thesensor interacts with the process fluid in order to make the desiredmeasurement. For example, spectroscopic instruments interact withprocess fluid through some type of optical window. Normal operation ofsuch sensors typically requires that the surface of the sensor beabsolutely free of contaminants, such as organic growth, solids, filmsor coatings, in order to take accurate measurements. For many processesthis requirement is difficult to achieve. Therefore, various methodshave been developed for cleaning the surfaces of such sensors.

Some methods for cleaning in-process sensors require removing the sensorfrom service, either by physical removal of the sensor from the processinstallation or by isolating (valving off) the sensor from the process.Both of these methods can be time consuming, especially if the sensorsurface fouls quickly. These methods are potentially dangerous, forexample, if the process involves toxic or otherwise hazardous chemicals.These methods may also harm the equipment. Moreover, the process itself,in addition to the process measurement, may be suspended until aftercleaning has been completed.

An upgrade to isolation of an in-process sensor for cleaning purposes isavailable in some systems, wherein a cleaning fluid is directed at thesensor during operation. These systems are limited to those where theprocess is not detrimentally affected by addition of the cleaning fluid.One example is in waste water treatment where clean, pressurized waterand/or air is directed at a sensor for cleaning purposes.

Mechanical methods have also been developed for cleaning of in-processsensors. Such methods involve use of wipers, brushes, and the like tophysically scrape contaminants off the sensor. Disadvantages of thesemethods include limited use with viscous process streams, the necessityto suspend the process measurement, and difficulty in designing amechanical cleaning device into the process equipment, especially for aprocess containing corrosive or otherwise hazardous streams.

Ultrasound has been applied to cleaning of in-process sensors. The useof ultrasound generates cavitation near the sensor to remove solids.However, ultrasound is limited to use with low solids and viscosityprocess streams, at pressures below 100 psig, certain temperatures, andstreams with low specific gravity.

U.S. Pat. Nos. 4,307,741 and 4,385,936 disclose an apparatus and aprocess for cleaning a probe inserted into a sample process stream. Theapparatus comprises a canister containing a cleaning agent, from whichthe cleaning agent is discharged, mixed with water and pumped through anozzle with a jet spray end directed at the probe. The method isdescribed as an improvement over cleaning methods that involve (1)removal of a probe from a process, (2) use of ultrasound to vibrateprocess fluid as it passes the probe and (3) use of brushes and/orwipers while the probe is in service. Nevertheless, the apparatus andmethod rely on introduction of a material (i.e., cleaning agent) foreignto the process stream.

U.S. Pat. No. 5,185,531 provides a cleaner for an in-line optical sensorcomprising a blade. The blade mechanically wipes the surface of a sensorwindow. Measurements from the sensor are suspended until the cleaningoperation is complete and data re-stabilizes.

German Patent Application DE 35 38 313 A1 discloses a device to cleansensors in bodies of water such as ditches around oil storage tanks,where the sensors are contaminated with animal or plant material. Thedevice is essentially a hose with a nozzle from which a pressurizedfluid is discharged so as to impact the external surface of the sensor.The pressurized fluid is water and/or air, which generates bubbles orcurrent in the water, creating an oscillatory motion at the surface ofthe sensor to remove contaminants.

All of the aforementioned methods include numerous limitations anddisadvantages. Therefore, it is desirable to have a cleaning system forin-process sensors, that allow for in situ cleaning of the sensorsurface while the sensor is in operation, that is, measurement need notbe suspended during cleaning. It would further be advantageous to cleanthe sensor without introducing foreign material, such as cleaning fluidsinto the process. The present invention meets these needs.

SUMMARY OF THE INVENTION

The present invention provides a cleaning system for an in-processsensor comprising:

-   -   (a) a sensor, wherein the sensor has a surface, which is in        contact with a process fluid;    -   (b) a scouring jet having an inlet and an outlet;        wherein the process fluid is in contact with said inlet of the        scouring jet and, wherein the jet is positioned relative to the        sensor such that discharge from the outlet impinges on the        surface of the sensor.

The present invention further provides a method for cleaning anin-process sensor, comprising:

-   -   (a) directing at least a portion of the process fluid to a        scouring jet; and    -   (b) discharging said fluid from the scouring jet such that the        fluid impinges on the sensor.

The improved cleaning system and method of the present invention areuseful in any process where there is present an in-process sensorsusceptible to contamination from the sensor's environment, includingcomponents of the process stream. The improved cleaning system andmethod are particularly useful for processes where sensors are exposedto high solids, viscous streams, toxic, corrosive, or otherwisehazardous streams where removal of the sensor, or even isolating thesensor may create dangerous conditions. Furthermore, the system andmethod are useful for processes that contain highly reactive orhazardous materials, and, where use of even relatively benign cleaningagents such as air and/or water are not possible. In such systems, useof the cleaning system and method are especially advantageous to providesafe, reliable measurements for both continuous and batch processes.

Advantages of the cleaning system and method of this invention include:

-   -   Continuous process operation and sensor operation during        cleaning of sensor.    -   Use of process fluid avoids introduction of a foreign material        into the process.    -   Cleaning system is wholly contained within process operations,        avoiding the need to remove the sensor to clean.    -   Process fluid is never contaminated with non-process cleaning        fluid.    -   Stringent control of composition of the process fluid is        maintained.    -   Viscous process fluids can be used.    -   Improved safety of operation when process fluid comprises        hazardous (toxic, corrosive, highly reactive) materials.    -   Sterile conditions are maintained when sensors are used in        biological and pharmaceutical systems.    -   High level of control over cleaning operation is available by        independently adjusting intensity, frequency and duration of        scouring.

The improved cleaning system and method are useful in a number ofindustries wherein in-process sensors are employed. A few examplesinclude chemical processes where the process fluid comprises corrosive,toxic or other hazardous components (e.g., titanium ore chlorination),processes in the pharmaceutical and food industries where carefulcontrol of the composition of the process fluid is important, if notcritical.

BRIEF DESCRIPTION OF THE FIGURE

FIGURE is a schematic diagram of a cleaning system of this invention foruse in a continuous process.

DETAILED DESCRIPTION

The present invention provides a system and method for cleaningin-process sensors. The present invention is extremely versatile,applicable in nearly all process environments where in-process sensorsare used for measurements of process conditions and compositions. Thecleaning system can be used in batch or continuous processes. Thecleaning system is not limited by temperature, pressure, or chemicalenvironment so long as appropriate equipment for such condition isutilized, which can readily be determined by those skilled in the art.Indeed, specific advantages of the cleaning system relate to its utilityin highly corrosive environments, wherein the fluid is highly viscousand erosive.

The cleaning system of this invention enables the removal ofcontaminants, such as organic growth, solids, gels, films or coatingscomprising gaseous or liquid phases, from the surface of an in-processsensor. An in-process sensor is defined herein as an analyticalinstrument, which interacts with a process fluid, resulting in ameasurement of one or more characteristics of the fluid and itscomponents. Furthermore, by “in-process” it is meant herein to encompasslarge manufacturing operations, such as processing plants for commoditychemicals, as well as small scale manufacturing operations, such asthose for fine chemicals and even laboratory scale syntheses. Examplesof characteristics of a process fluid capable of being measured includeelectrical characteristics such as pH or conductivity and theconcentration of one or more chemical components of the fluid. Thesensor may also be part of a vessel through which the instrumenttransmits, such as a nuclear magnetic level device. This invention isparticularly advantageous when used to clean sensors which measureconcentrations of at least one component of the process fluid.

Examples of in-process sensors suitable for use with the cleaning systemof this invention include electrodes, such as pH meters and dissolvedoxygen analyzers, membranes, nuclear density and level gauges, andspectroscopic instruments, including Infrared, Raman, and Near Infraredinstruments and particle size detectors. The spectroscopic instrumentsmay use cells or probes. The preferred sensor for any particularapplication will depend on the process, that is, the equipment beingused, operating conditions (temperature, pressure), the characteristicbeing measured, chemical composition, reactor vessel design, etc.

The cleaning system of this invention is particularly advantageous whenInfrared, Raman or Near Infrared spectroscopic instruments are used forcomposition measurements. When such instruments are used, probes arepreferred over cells. In a particular application, for compositionmeasurements of a process fluid in the chlorination of titanium ores,use of an Infrared spectroscopic probe is preferred. More preferably,for this application, the probe is a Fourier transform infrared probe(FTIR), and most preferred is a FTIR attenuated total reflectance (ATR)probe. Preferably the ATR probe has a diamond or sapphire window exposedto the process fluid. For use with process fluids from the chlorinationof titanium ores, the preferred window is a diamond window, whichintroduces less interference in the desired spectroscopic range.

In the cleaning system of this invention, the sensor has a surface whichis in contact with a process fluid. “Surface” of the sensor is meantherein to include windows of spectroscopic instruments as well aswindows on reaction vessels through which an electromagnetic signal maypass to generate a measurement, electrode surfaces, cell walls, and thelike. Contact of the process fluid with the sensor may occur by passingthe fluid through a pipe upon which the sensor, such as a spectroscopicprobe, is mounted, or by inserting the sensor, such as an electrode,into the process fluid in a reaction vessel, for example through a valveor flange. The sensor may contact the process fluid through a port in amain reaction vessel, such as a stirred tank or a pipe or continuousstirred tank reactor (CSTR) in a continuous process. Frequently, thesensor will contact the process fluid through a side stream from a mainprocess fluid, for example through a sampling system.

By “process fluid” it is meant herein the contents of a reaction vessel.The process fluid may comprise a liquid and/or a gas and often willcontain entrained solids. In a continuous process, the process fluid maybe in the form of a process stream flowing through a pipeline reactor orone or more of a series of continuous stirred tank reactors. The processfluid may be in the form of solution or slurry in a batch reactionvessel, such as a stirred tank. In a batch process, there is furtherprovided a pump, which may be located within a batch reaction vessel orexternal to the vessel through which to transfer process fluid to thejet.

The scouring jet of this invention is a vessel, such as a pipe, fromwhich a fluid is emitted wherein the fluid, upon being emitted, expandsinto the environment immediately surrounding the outlet of the jet anddecelerates. The scouring jet has an inlet in contact with the processfluid and through which the fluid enters the jet and an outlet fromwhich the fluid is emitted. The jet inlet may be in contact with theprocess fluid directly from the reaction vessel. More commonly, andpreferably, a side stream from the reaction vessel is used to direct aportion of the process fluid to the jet inlet.

It should be recognized that the primary function of the fluid directedto and emitted from the scouring jet is to clean the sensor surface.Even though the jet directs fluid toward the sensor, and the sensor maymeasure a characteristic of this fluid, a separate flow of fluid,different from the jet flow is generally the main source of measurementfor the sensor.

The outlet of the scouring jet may be comprised of a single nozzle ormultiple nozzles from which the fluid is emitted. The jet outlet issmaller in diameter than the vessel into which the jet discharges thefluid. Decreasing the size (diameter) of the outlet, results inincreasing the velocity of flow from the jet. The shape of the outletnozzle(s) may be designed in any shape suitable to clean the surface ofthe sensor. For example, a small single optical window or electrode maybe effectively cleaned by a nozzle having a small round shape. A largeoptical window or membrane may be effectively cleaned by a nozzle with alarge diameter, a slot-shaped nozzle, or even multiple smallround-shaped nozzles.

The scouring jet is positioned relative to the sensor such thatdischarge of the process fluid from the jet outlet impinges on thesurface of the sensor in contact with the process fluid. The distance ofthe jet outlet from the sensor depends on the conditions into which thefluid is emitted and the geometry and dimensions of the jet, includingthose of the jet outlet, angle of emission from the jet relative to thesensor and relative to a process flow direction, and others. Suchparameters can be optimized using standard engineering calculations. Inone particular application, that is, wherein the sensor measurescomposition of a process fluid in chlorination of titanium ores, the jetoutlet is positioned between 0.25 and 1.0 inches from the sensorsurface.

The angle at which the jet flow impinges on the sensor surface is in therange of 0° to 90°, where 0° is parallel to the sensor surface and 90°is perpendicular to the sensor surface, preferably 45° to 90°. Thepreferred angle and distance between the jet and the sensor may bedetermined experimentally or by calculations so that for a givengeometry and velocity the desired degree of cleaning will be achieved.Harder contaminants and smaller sensor surfaces are better suited toimpingement at higher angles, that is, perpendicular to the sensorsurface. Softer contaminants and larger sensor surfaces may be bettersuited to impingement at lower angles, that is, more parallel to thesurface. When the sensor is a flow-through spectroscopic cell, where aperpendicular jet would interfere with the spectroscopic light beam,placing the jets at an angle less than 90° is desirable, to limit jetinterference with equipment surrounding the sensor.

It should be recognized that flow of fluid through the jet need not becontinuous. Valves may be positioned to permit or prevent or controlflow of fluid through the jet. For example, a valve may isolate theprocess fluid from the sensor, if it is desired to discontinue themeasurement during the cleaning process. Alternatively, a valve mayisolate the scouring jet from the process fluid, in order to operate thecleaning system on a less than continuous, that is, periodic, basis.Furthermore, volumetric flow of process fluid through the jet may becontrolled by using valves. Such valves may be operated manually orpreferably, automatically. The valves selected should provide goodcontrol of flow and be durable. For example, for use with process fluidscomprising solid particulates, erosion resistant valves, such as thosewith ceramic liners are effective.

Process equipment surrounding the sensor is preferably made frommaterials that are erosion resistant. If easily abraded materials havebeen used, such materials are preferably coated with hard-facing orshielded with a hard, erosion resistant material. In addition, the pipeor vessel into which the sensor is inserted is preferably sufficientlylarge so the velocity of fluid exiting the jet can drop rapidly afterimpact with the sensor in order to minimize potential for damage toequipment surrounding the sensor which may be impacted by the jet flowdeflected after impact with the sensor.

During operation of the cleaning system of this invention, movement ofthe process fluid through the system may induce vibration due to designof the scouring jet, sensor and process lines through which the fluidpasses. This vibration may be detrimental to measurement accuracy.Preferably means should be take to avoid the vibration such as,stiffening the jet, bracing the sensor chamber and placing flowobstructions, such as valves and elbows in the process vessels at adistance from the sensor and jet. Preferably any obstruction is locatedat least 10 diameter measures upstream and 5 or more diametersdownstream of the jet.

The method of this invention comprises directing at least a portion of aprocess fluid to a scouring jet. It should be recognized that the flowof process fluid to the jet can be controlled independently from theflow of fluid to the sensor. The portion of the process fluid directedto the jet can be as much as 100% of the entire process fluid flow, butpreferably is less than 50%. Directing 100% of the process fluid flow tothe jet may be advantageous when the sensor surface is so contaminatedthat the objective is to clean the surface until the contamination issufficiently reduced and/or eliminated. When a portion of the processfluid is directed to the jet, the portion is typically separated fromthe main process fluid as a side stream, especially using sample systemprinciples. The portion of the fluid directed to the jet is dischargedfrom the jet such that the fluid impinges on the sensor surface, therebycleaning the surface. Both the velocity of the jet as well as anysuspended solids present in the process fluid provide scouring action toclean the sensor.

The method of this invention may be performed continuously duringoperation of the sensor, or intermittently, based on a periodicschedule, especially if the rate of fouling is well understood, or on anas-needed basis, that is, when fouling of the sensor is either suspectedor detected. The method of this invention may be controlled with respectto duration of the cleaning, if cleaning is not performed continuously,as well as with respect to intensity of the cleaning, such as due tovelocity of the fluid discharged from the jet.

The duration of a cleaning cycle, when intermittent cleaning ispracticed may be from as short as a few seconds to many hours. Thefraction of time cleaning occurs during process operation may be lessthan 1% of the time to as much as 100% of the time. To minimizepotential for erosion of the sensor surface, preferably duration ofcleaning should continue only as long as necessary to remove thecontaminants.

The intensity of the process fluid exiting the jet should be sufficientto remove the contaminants, that is, to clean the sensor surface.However, consideration must be given to minimize damage to the sensorand its surrounding environment. Intensity is a function of velocity ofthe jet, in addition to the characteristics of the process fluid,distance of the jet outlet from the sensor and other factors, including,for example, hardness of particulate materials in the process fluid.

The preferred jet velocity may range from a few inches per second toseveral hundred feet per second. In operation, the preferred velocitywill depend on the composition and characteristics of the process fluid,distance of the jet outlet from the sensor, and other factors such ashardness of the contaminant, durability of the sensor surface, which canbe determined by experimental optimization.

It may be desirable to treat the portion of the process fluid from themain process fluid. Such treatment may involve, for example, heating orcooling the process fluid, vaporizing a condensed process fluid, orremoving or adding particulate matter to the fluid. It may beparticularly desirable to remove particulate matter that may beparticularly abrasive. Particulate matter may be removed, for example,by means such as filters, cyclone separation. However, it should berecognized there is reduced scouring absent particulates in the fluid.

In a preferred embodiment of this invention, the cleaning system of thisinvention further comprises a control strategy. The control strategy mayprovide for independent adjustment of the intensity, frequency andduration of the cleaning operation. Thus, a high level of control overthe cleaning system is provided. The sensor may provide a measurement ofa characteristic of a contaminant that obstructs or obscures the sensorin addition to measuring the desired characteristic of the processfluid. Spectroscopic probes are the preferred sensors for this mode ofoperation, more preferably Infrared probes, and most preferably, FTIRATR probes. In one example of a control strategy, the sensor provides ameasurement of a characteristic of the contaminant, wherein themeasurement relates to the concentration of the contaminant, andcompares the measurement with a predetermined set point. Operation ofthe scouring jet is modified in a manner to maintain or reduce theconcentration of solids or contaminants below the set point.

An important advantage of this process is that the cleaning method canbe operated while the sensor continues to monitor the process andperform the measurements. This is particularly useful when the sensor isbeing used in classical continuous feedback control operation.

The cleaning system and method of this invention has particular utilityin process operations, using in-process spectroscopic probes, especiallyInfrared probes, such as the process described in U.S. patentapplication Ser. No. 09/739,597, filed Dec. 18, 2000, the teachings ofwhich are hereby incorporated by reference. For example, in the processfor making titanium dioxide, the present invention may be used in thechloride or the sulfate process to control or monitor steps in theprocess including controlling or monitoring in chlorination, oxidation,finishing or in precipitation and finishing, respectively.

In addition, manufacturing processes in the pharmaceutical andbiological materials industries are particularly suitable to include theuse of the present invention. For example, the present invention isdirectly applicable to these and other processes where maintaining theintegrity of the process stream is a consideration. Integrity may beparamount for a process that cannot tolerate the smallest contaminant orto protect operators from contact with a process that may contain highlytoxic, such as carcinogenic or mutagenic, materials or organisms.

EXAMPLES Example 1

The following specific example describes in detail a typicalinstallation and operation of the cleaning system and cleaning method ofthis invention for a continuous process operation, with reference to theFIGURE. The arrows within the piping in the FIGURE indicate thedirection of flow of the process fluid. In this example, a scouring jetis used to clean the window of an attenuated reflectance FTIR probemeasuring impurities in a crude titanium tetrachloride stream inaccordance with CH2626.

A process fluid (1) derived from a fluid bed chlorinator, whichcomprised a mixture of metal chlorides and oxychlorides and suspendedsolid particles in a process to manufacture titanium tetrachloride wasfed to a reaction vessel (2). This fluid was treated with water topassivate the aluminum chloride present (not shown) and the treatedfluid (3) was transferred using pump (4) from vessel (2) to main processflow (6).

A portion of the treated fluid (5) was removed from the main processflow (6). With valve (7) open, a main sample flow of treated processfluid (8) was directed to an Infrared analyzer (9) as a side stream. Theanalyzer comprised a diamond window (10) which contacted the processfluid (8) and recorded a measurement, which was transmitted to afeedback controller (not shown) to control the amount of water added(not shown).

A secondary flow of treated process fluid (11) was directed through openvalve (12) to a scouring jet (13). Ceramic valves (7) and (12) werecontrolled using a feedback loop to adjust flow between main sample flow(8) and flow to scouring jet (11). Fluid (11) exited scouring jet (13)through a nozzle (14), which was round in shape and directed at an angleof 90° and a distance of between 0.25 and 1.0 inches toward window (10)of analyzer (9), resulting in a scouring effect to remove solidparticulates and generating combined sample flow (15), which was fedinto reaction vessel (2).

Example 2

The following specific example describes in detail a typicalinstallation and operation of the cleaning system and cleaning method ofthis invention for a batch process operation.

A Fourier Transform Infrared probe is placed in a recycle loop on astirred tank reaction vessel used in a fermentation process where thefermentation mixture is the process fluid. A recycle pump is used toremove fluid from the reaction vessel, cycle through a sample systemwhere spectroscopic measurements are made to monitor the composition ofthe process fluid. A side stream is removed from the recycle loop and isfed to a scouring jet. The process fluid is discharged from the scouringjet with sufficient kinetic energy, derived from the pressure and flowrate generated by the pump, to clean the spectroscopic probe.

Example 3

The cleaning system and method of this invention can be applied toanalysis of fluid using flow-through infrared analyzer cell. A fluidundergoes Infrared spectroscopic analysis in a standard flow throughcell. The fluid sample flows in the bottom of the cell, and exitsthrough the top of the cell. Windows on either side of the cell permitthe Infrared beam to enter one window, pass through the cell and exitthe opposite window to a detector. Solids in the stream adhere to thewindows requiring continuous removal to permit accurate analysis of thefluid composition.

A pressurized stream of the fluid being analyzed is supplied to twoscouring jets entering the sample cell below each window and oriented sothat they impact the window at an angle of approximately 45°. This anglepermits the jets to be positioned outside the infrared beam path.Velocity through the jets is controlled by manual adjustment of valvesin the lines leading to each jet. Optimum velocity is determinedexperimentally, by slowly opening the valves until the solids coatingthe window are removed.

Example 4

The cleaning system and method of this invention can be applied toanalysis of a liquid in a tank using a pH probe using a single jet toclean both a glass and reference electrode. A pH probe is insertedthrough a port in the side wall of an agitated tank containing a liquidin which a batch chemical reaction is taking place, producing gelatinoussolids, which collect on the surface of the pH probe electrodes. Thesolids on the electrodes cause errors in measurements by the pH probe ofthe liquid in the tank as the reaction takes place.

The glass and reference electrodes of the pH probe are both flatsurfaces and are adjacent, together covering an area of about 0.75inches in diameter.

In order to clean the surfaces of the electrodes, liquid is withdrawnfrom the tank into a centrifugal pump with a variable-speed drive. Theliquid exiting the pump flows through a pipe passing through the sameport used to insert the pH probe into the tank. The flow exits through ascouring jet nozzle at the end of this pipe at an angle of approximately5°, that is, nearly parallel to the electrode surfaces, and the flowimpinges upon the surfaces of the electrodes.

The speed of the pump is increased to increase scouring jet velocity,and thus scouring intensity increases as the reaction proceeds and thegeneration of solids increases. The speed necessary to provide adequatecleaning at each phase of the reaction is determined experimentally.Once an acceptable pump speed profile is determined, the same profile isused for each subsequent time this particular chemical reaction isperformed in the tank.

The Examples provided herein are illustrative and should not beconstrued as limiting the scope of the invention. Variations will bereadily appreciated by those skilled in the art.

1. A cleaning system for an in-process sensor comprising: (a) a reactionvessel having an interior region and having a process fluid locatedtherein; (b) a sensor having a surface, wherein the surface of thesensor is positioned on the interior region of the reaction vessel suchthat the surface of the sensor is capable of contacting the processfluid and having a build up of the contaminants thereon, and wherein theregion of the reaction vessel surrounding the sensor is coated withhard-facing material; and (c) a scouring jet having an inlet in contactwith the process fluid and through which the process fluid enters thejet and an outlet from which the process fluid is emitted, the outlet ofthe scouring jet being positioned so that the process fluid is emittedin a direction toward the surface of the sensor to impinge on thesurface of the sensor whereby the impinging process fluid has a scouringaction capable of removing one or more of the contaminants from thesurface of the sensor; and wherein the process fluid emitted from thescouring jet does not contain a cleaning fluid.
 2. The cleaning systemof claim 1 wherein the reaction vessel is a closed reaction vesselselected from the group consisting of a tank and a pipe.
 3. The cleaningsystem of claim 1 further comprising one or more valves located betweenthe inlet to the scouring jet and the outlet of the scouring jet.
 4. Thecleaning system of claim 1 wherein the sensor is an Infrared, Raman orNear Infrared spectroscopic instrument.
 5. The cleaning system of claim4 wherein the instrument has a probe.
 6. The cleaning system of claim 5wherein the probe has a window in contact with the process fluid.
 7. Thecleaning system of claim 4 wherein the sensor is an Infraredspectroscopic instrument.
 8. The cleaning system of claim 1 wherein thesensor surface is positioned in the reaction vessel by way of a port,valve or flange.
 9. A cleaning system for an in-process sensor for usein a process for making titanium dioxide by either the chloride orsulfate processes comprising: (a) a reaction vessel having an interiorregion and having a process fluid located therein, wherein the processfluid is from either the chloride or sulfate processes; (b) a sensorhaving a surface, wherein the surface of the sensor is positioned on theinterior region of the reaction vessel such that the surface of thesensor is capable of contacting the process fluid and having a build upof the contaminants thereon, and wherein the region of the reactionvessel surrounding the sensor is coated with hard-facing material; and(c) a scouring jet having an inlet in contact with the process fluid andthrough which the process fluid enters the jet and an outlet from whichthe process fluid is emitted, the outlet of the scouring jet beingpositioned so that the process fluid is emitted in a direction towardthe surface of the sensor to impinge on the surface of the sensorwhereby the impinging process fluid has a scouring action capable ofremoving one or more of the contaminants from the surface of the sensor;and wherein the process fluid emitted from the scouring jet does notcontain a cleaning fluid.
 10. The cleaning system of claim 9 furthercomprising one or more valves located between the inlet to the scouringjet and the outlet of the scouring jet.
 11. The cleaning system of claim9 wherein the sensor is an Infrared, Raman or Near Infraredspectroscopic instrument.
 12. The cleaning system of claim 11 whereinthe instrument has a probe.
 13. The cleaning system of claim 12 whereinthe probe has a window in contact with the process fluid.
 14. Thecleaning system of claim 9 wherein the sensor is an Infraredspectroscopic instrument.